Method and device for converting thermal energy of a low temperature heat source to mechanical energy

ABSTRACT

In a method and device ( 1 ) for converting thermal energy of a low temperature heat source ( 20 ) into mechanical energy in a closed circuit, a liquid working agent is heated by transmitting heat from the low temperature source ( 20 ) and partially evaporating it in an expansion device ( 3 ). Erosion to the condenser ( 8 ) for condensing the partially evaporated working agent can be prevented by separating the liquid phase from the evaporator phase in the partially evaporated working agent that is directly in front of the condenser ( 8 ), and only the evaporator phase is transferred to the condenser ( 8 ) for condensing and subsequently, the condensed evaporator phase and the liquid phase are merged.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2007/062147 filed Nov. 9, 2007, which designatesthe United States of America, and claims priority to German ApplicationNo. 10 2007 041 457.0 filed Aug. 31, 2007, the contents of which arehereby incorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to a method and apparatus for conversion of heatenergy from a low-temperature source to mechanical energy.

BACKGROUND

A method such as this and an apparatus such as this are known, forexample, from U.S. Pat. No. 7,093,503 B1.

In order to use the heat energy from low-temperature heat sources, forexample geothermal sources, gas, vapor or liquid waste-heat sources orsolar energy, it is already known for an agent in a circuit not to bevaporized by the heat source, but only to be heated. As a result of thelack of vaporization, the heat energy which is normally required tovaporize the agent can be used, for example, to heat the considerablygreater mass flow of the agent. This makes it possible to achieveconsiderable efficiency advantages over circuits in which the agent isvaporized, for low-temperature sources in the temperature range below400° C.

In the case of a circuit which is known from U.S. Pat. No. 7,093,503 B1,in a first step, a liquid agent is raised to an increased pressure by apump. In a second step, the increased-pressure, liquid agent is heatedin a heat exchanger by heat transfer from a low-temperature source. In athird step, the heated, liquid agent is expanded in a two-phase turbine,with an expanded, partially vaporized agent with a liquid phase and avapor phase being produced by partial vaporization of the agent, andwith heat energy in the agent being converted to mechanical energy.

The two-phase turbine for this purpose has nozzles directly adjacent toits inlet, in which the agent is expanded by increasing its volume froma relatively high inlet pressure to a lower outlet pressure, as a resultof which the agent is partially vaporized. The water-steam jet which iscreated in this way is passed to turbine blades of the turbine, by meansof which the kinetic energy of the water-steam jet is converted tomechanical energy of a rotor shaft. The rotor shaft is in turn connectedto a generator, via which the mechanical energy of the rotor shaft isconverted to electrical energy.

The two-phase agent leaving the turbine is then supplied to a condenser.In a fourth step, the vapor phase of the expanded, partially vaporizedagent is then condensed in the condenser, thus producing the initiallymentioned liquid agent. This is supplied to the pump that has alreadybeen mentioned, thus closing the circuit. The T-s-diagram illustrated inFIG. 2 shows the circulating process which takes place in this case. Inthis case, SL denotes the boiling line, TL the dew line and K thecritical point of the agent. The agent is heated along the boiling lineSL from the point A to the point B in the vicinity of the critical pointK, is expanded, being partially vaporized, from point B to point C, andis condensed from point C to point A.

It is furthermore known from WO 2005/031123 A1 for a two-phase mixtureleaving a two-phase turbine to be supplied to a separator in order toseparate the vapor phase from the liquid phase. The vapor phase is thenexpanded further in a steam turbine in order to produce additionalmechanical energy. The expanded steam leaving the steam turbine issupplied to a condenser in which it is condensed, is then raised to anincreased pressure by means of a pump, and is then combined with theliquid phase, which has been separated in the separator, of thetwo-phase mixture. The agent flow created in this way is then pumpedinto a heat exchanger with the aid of a further pump, by being heated byheat transfer from a low-temperature source. The condenser is thereforesupplied only with the exhaust steam from the steam turbine, but notwith the two-phase mixture of the two-phase turbine. Although thiscircuit is distinguished by very high efficiency, it is, however, alsodistinguished by being considerably more complex and by involvingconsiderably greater investment costs.

In the case of a circuit which is known from EP 0 485 596 A1, only oneheated liquid, that is to say not vaporized, agent is likewise suppliedto an expansion device, in which it is partially vaporized. Thewater-steam mixture leaving the expansion device is then supplied to aseparator, which is used only to measure the liquid components in thesteam.

If the two-phase mixture leaving the turbine is supplied to thecondenser in the initially mentioned circuit, then the liquid componentscan lead to erosion of the condenser, thus shortening the life of thecondenser.

SUMMARY

According to various embodiments, a method and an apparatus can bedeveloped such that it is possible to reliably prevent erosion of thecondenser, without significantly increasing the complexity of thecircuit.

According to an embodiment, a method for conversion of heat energy froma low-temperature heat source to mechanical energy in a closed circuitmay comprise the following steps:

step 1: increasing the pressure of a liquid agent,

step 2: heating of the increased-pressure, liquid agent by transferringheat from the low-temperature heat source to the agent, withoutvaporizing the agent,

step 3: expanding the heated, liquid agent, wherein an expanded,partially vaporized agent with a vapor phase and a liquid phase isproduced by partial vaporization of the agent, and heat energy in theagent is converted to mechanical energy,

step 4: condensing the vapor phase produced in step 3 in a condenser inorder to produce the liquid agent from step 1, wherein

in the case of the expanded, partially vaporized agent produced in step3, the liquid phase is separated from the vapor phase immediately beforethe condenser

only the vapor phase is supplied to the condenser,

the condensed vapor phase and the liquid phase are combined after thecondenser but before step 1, in order to produce the liquid agent.

According to a further embodiment, the pressure of the agent in thecondenser can be set to an optimum between the droplets of the liquidphase in the vapor phase of the agent being as small as possible and themechanical energy produced being as great as possible in step 3.According to a further embodiment, the condensed vapor phase and theliquid phase can be combined in an agent reservoir. According to afurther embodiment, the low-temperature source can be at a temperatureof less than 400° C.

According to another embodiments, an apparatus for conversion of heatenergy from a low-temperature heat source to mechanical energy in aclosed circuit, may comprise a pump for increasing the pressure of aliquid agent, a heat exchanger for heating the increased-pressure,liquid agent by transferring heat from the low-temperature heat sourceto the agent, without vaporizing the agent, an expansion device forexpanding the heated, liquid agent, wherein an expanded, partiallyvaporized agent with a liquid phase and a vapor phase can be produced bypartial vaporization of the agent in the expansion device, and heatenergy in the agent can be converted to mechanical energy, a condenserfor condensation of the vapor phase of the partially vaporized agent inorder to produce the liquid agent, a separator for separation of theliquid phase from the vapor phase of the expanded, partially vaporizedagent, wherein the separator is arranged immediately before thecondenser in the flow direction of the agent, and is connected to thecondenser in order to supply the vapor phase to the condenser, and acombination means for combining the liquid phase and the condensed vaporphase of the partially vaporized agent, wherein the combination means isarranged before the pump in the flow direction of the agent and isconnected to the separator in order to supply the liquid phase, and tothe condenser in order to supply the condensed vapor phase, to thecombination means.

According to a further embodiment, the pressure of the agent in thecondenser can be set to an optimum between the droplets of the liquidphase in the vapor phase of the agent being as small as possible and themechanical energy produced being as great as possible in the expansiondevice. According to a further embodiment, the combination means can bein the form of an agent reservoir. According to a further embodiment, anozzle and a turbine can be arranged successively in the flow directionof the agent in the expansion device. According to a further embodiment,the nozzle and the turbine may form a single physical unit. According toa further embodiment, the low-temperature source can be at a temperatureof less than 400° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention as well as further refinements will be explained in moredetail in the following text with reference to exemplary embodiments inthe figures, in which:

FIG. 1 shows a simplified, schematic illustration of a circuit for anapparatus according to an embodiment, and

FIG. 2 shows a T-s-diagram of a circuit known from the prior art with anagent being heated (without vaporization) by a low-temperature source.

DETAILED DESCRIPTION

The method according to various embodiments provides that in the case ofthe expanded, partially vaporized agent, the liquid phase is separatedfrom the vapor phase immediately before the condenser. Only the vaporphase is supplied to the condenser for condensation. The condensed vapor(that is to say then liquid) phase and the separated liquid phase arecombined after the condenser but before step 1, that is to say theincrease in the pressure of the liquid agent, in order to produce theliquid agent. The liquid phase therefore bypasses the condenser, thusmaking it possible to prevent erosion of the condenser. All that isrequired for this purpose is a separator for separation of the liquidphase from the vapor phase, a bypass line for the liquid phase line tobypass the condenser and a combination means for combining the(separated) liquid and condensed vapor (that is to say then liquid)phase. The complexity of the circuit is therefore increased onlyinsignificantly.

The size of the droplets in the liquid phase in the vapor phase of theagent after expansion is dependent on the pressure of the agent in thecondenser. The higher the pressure of the agent is in the condenser, andthus at the outlet of the expansion device, the smaller the droplets. Inturn, the smaller the droplets are, the less is the risk of erosioncaused by the droplets. On the other hand, however, as the pressure ofthe agent in the condenser and at the outlet of the expansion deviceincreases, the mechanical energy which can be produced by conversion ofheat energy by the expansion device decreases.

Preferably, therefore, the pressure of the agent during the condensationprocess is set to an optimum between the droplets of the liquid phase inthe vapor phase of the agent being as small as possible and themechanical energy produced being as great as possible in step 3. Theamount of mechanical energy produced is therefore deliberately reducedin order to prevent erosion of the condenser. Because of the enormousefficiency advantage resulting from heating rather than vaporization ofthe agent by the low-temperature heat source, however, considerableefficiency advantages can nevertheless still be achieved in comparisonto conventional circuits in which the agent is vaporized by thelow-temperature heat source.

According to one embodiment of the method, the condensed vapor (that isto say then liquid) phase and the (separated) liquid phase are combinedin an agent reservoir. Since a reservoir such as this is provided in anycase in many circuits, there is no need for an additional component forcombination of the two phases.

In this case, particularly high efficiencies can be achieved if thelow-temperature source is at a temperature of less than 400° C.

The apparatus according to various embodiments has a separator forseparation of the liquid phase from the vapor phase of the expanded,partially vaporized agent, wherein the separator is arranged immediatelybefore the condenser in the flow direction of the agent. A combinationmeans is used to combine the (separated) liquid phase and the condensedvapor (that is to say then liquid) phase of the expanded, partiallyvaporized agent, wherein the combination means is arranged before thepump in the flow direction of the agent. The separator is connected tothe condenser in order to supply the vapor phase to the condenser. Thecombination means is connected to the separator in order to supply the(separated) liquid phase to the combination means, and is connected tothe condenser in order to supply the condensed vapor (that is to saythen liquid) phase to the combination means. The advantages that havebeen mentioned for the method according to various embodiments apply ina corresponding manner to the apparatus.

The pressure of the agent in the condenser can preferably be set to anoptimum between the droplets of the liquid phase in the vapor phase ofthe agent being as small as possible and the mechanical energy producedbeing as great as possible in the expansion device.

According to one embodiment, the combination means is in the form of anagent reservoir.

Advantageously, a nozzle and a turbine can be arranged successively inthe flow direction of the agent in the expansion device in order toexpand the heated agent. The agent can be expanded in the nozzle byincreasing its volume from a higher inlet pressure to a lower outletpressure, thus partially vaporizing the agent. The water-steam jet whichis created in this way can then be passed to the turbine blades of theturbine, by means of which the kinetic energy of the water-steam jet isconverted to mechanical energy of a rotor shaft. Instead of only asingle nozzle, a plurality of nozzles can also be arranged at theturbine inlet, for example in an annular configuration, through whichthe agent can flow in parallel.

In this case, the nozzle and the turbine may also form a single physicalunit, that is to say the nozzles are arranged directly adjacent to theturbine inlet.

An apparatus 1 according to various embodiments for conversion of theheat energy of a low-temperature heat source to mechanical energycomprises a thermodynamic circuit in which a heat exchanger 2, anexpansion device 3, a separator 7, a condenser 8, an agent reservoir inthe form of a condensate tank 9 and a pump 10 are arranged successivelyin the flow direction of an agent.

The low-temperature heat source is a heat source at a temperature ofless than 400° C. By way of example, heat sources such as these aregeothermal sources (hot thermal water), industrial waste-heat sources(for example waste heat from plants used in the steel, glass or cementindustries) and solar energy.

By way of example, a coolant liquid of the R134 type may be used as anagent for temperatures of less than 300° C., and, for example, a coolingliquid of the R245 type may be used for temperatures of more than 300°C. The pump 10 is used to pump the liquid agent to an increasedpressure.

The heat exchanger 2 is used to heat the increased-pressure, liquidagent in the circuit by heat transfer from the low-temperature heatsource 20 to the agent without vaporization of the agent, that is to saythe agent is only heated and is not vaporized in the heat exchanger 2.For this purpose, the low-temperature heat source 20, for example hotgeothermal water flows through the primary side of the heat exchanger,and the increased-pressure agent flows through its secondary side. Aline 11 connects the secondary side of the heat exchanger 2 to theexpansion device 3. The agent is still liquid at the outlet on thesecondary side of the heat exchanger 2, when it enters the line 11.

The expansion device 3 is used to expand the heated liquid agent,wherein an expanded, partially vaporized agent with a liquid and a vaporphase can be produced by partial vaporization of the heated liquid agentin the expansion device 3, and heat energy in the heated liquid agentcan be converted to mechanical energy. The expansion device 3 for thispurpose comprises a nozzle 4 and a turbine 5, which are arrangedsuccessively in the flow direction of the agent. The nozzle and theturbine may in this case form a single physical unit, that is to say thenozzle 4 is arranged immediately adjacent to the inlet of the turbine 5.Instead of only a single nozzle 4, it is also possible to arrange aplurality of nozzles 4 at the inlet of the turbine 5, for example in anannular configuration, through which the agent can flow in parallel.

On the outlet side, the turbine 5 is connected via a line 12 to theseparator 7. The separator 7 is used to separate the liquid phase fromthe vapor phase of the agent which has been partially vaporized in theexpansion device 3. The separator 7 is arranged immediately before thecondenser 8 in the flow direction of the agent, is connected via a line13 to the condenser 8 in order to supply the vapor phase to thecondenser 8, and is connected via a line 14 to the condensate tank 9 inorder to supply the liquid phase to the condensate tank 9.

The condenser 8 is used to produce the liquid agent by condensation ofthe partially vaporized agent.

The condensate tank 9 is used to combine the liquid phase and thecondensed vapor (that is to say then liquid) phase of the partiallyvaporized agent. The condensate tank 9 is arranged after the condenser 8and before the pump 10 in the flow direction of the agent, is connectedvia a line 14 to the separator 7 in order to supply the liquid phase,and via a line 15 to the condenser 8 in order to supply the condensedvapor phase to the condensate tank 9.

During operation of the apparatus 1, in a first step, liquid agent fromthe condensate tank 9 is raised to an increased pressure by the pump 10,and is pumped into the heat exchanger 2.

In a second step, the increased-pressure, liquid agent is heated,without being vaporized, in the heat exchanger 2 by transfer of heat tothe agent from the low-temperature heat source 20 which flows throughthe primary side of the heat exchanger 2.

In a third step, the heated, liquid agent is expanded in the expansiondevice 3, with the agent being partially vaporized and its heat energybeing converted to mechanical energy. The expansion device 3 thereforeproduces an expanded, partially vaporized agent with a liquid phase anda vapor phase. For this purpose, the heated, liquid agent which issupplied to the nozzle 4 via the line 11 is expanded in the nozzle 4 andin the process is partially vaporized. The kinetic energy of thewater-steam jet created in this way is converted in the turbine 5 intomechanical energy of a rotor shaft, and a generator 6 is thus driven,which in turn converts the mechanical energy to electrical energy.

The expanded, partially vaporized agent which is produced in the thirdstep and leaves the turbine 5 in the form of a two-phase mixture(steam/liquid) is supplied via a line 12 to the separator 7, in that thevapor phase is separated from the liquid phase of the two-phase mixture.

Only the vapor phase is supplied to the condenser 8 via the line 13. Inthe condenser 8, the vapor phase is condensed by cooling, for example bydirect cooling, air cooling, hybrid cooling or water cooling, and thecondensed vapor (that is to say then liquid) phase is supplied via theline 15 to the condensate tank 9.

The separated liquid phase, in contrast, bypasses the condenser 8 viathe line 14 and only after this, but still before the pump 10 andtherefore before the first step, is combined with the condensed vapor(that is to say then liquid) phase in the condensate tank 9.

Liquid agent from the condensate tank 9 is raised to an increasedpressure with the aid of the pump 10 and is pumped into the heatexchanger 2, thus closing the circuit.

Erosion of the condenser 8 can be prevented by separation of the liquidphase from the vapor phase of the two-phase mixture leaving the turbine5, in the separator 7, and by the liquid phase then being fed directlyinto the condensate tank 9, bypassing the condenser 8.

The pressure of the agent in the condenser 8 is in this case set to anoptimum between the droplets of the liquid phase in the vapor phase ofthe agent being as small as possible and the mechanical energy producedbeing as great as possible in the third step. This makes it possible toreduce the erosion of the condenser even further.

1. A method for conversion of heat energy from a low-temperature heatsource to mechanical energy in a closed circuit comprising the followingsteps: a) increasing the pressure of a liquid agent, b) heating of theincreased-pressure, liquid agent by transferring heat from thelow-temperature heat source to the agent, without vaporizing the agent,c) expanding the heated, liquid agent, wherein an expanded, partiallyvaporized agent with a vapor phase and a liquid phase is produced bypartial vaporization of the agent, and heat energy in the agent isconverted to mechanical energy, d) condensing the vapor phase producedin step c) in a condenser in order to produce the liquid agent from stepa), wherein in the case of the expanded, partially vaporized agentproduced in step c), the liquid phase is separated from the vapor phaseimmediately before the condenser only the vapor phase is supplied to thecondenser, the condensed vapor. phase and the liquid phase are combinedafter the condenser but before step a), in order to produce the liquidagent.
 2. The method according to claim 1, wherein the pressure of theagent in the condenser is set to an optimum between the droplets of theliquid phase in the vapor phase of the agent being as small as possibleand the mechanical energy produced being as great as possible in stepc).
 3. The method according to claim 1, wherein the condensed vaporphase and the liquid phase are combined in an agent reservoir.
 4. Themethod according to claim 1, wherein the low-temperature source is at atemperature of less than 400° C.
 5. An apparatus for conversion of heatenergy from a low-temperature heat source to mechanical energy in aclosed circuit, comprising a pump for increasing the pressure of aliquid agent, a heat exchanger for heating the increased-pressure,liquid agent by transferring heat from the low-temperature heat sourceto the agent, without vaporizing the agent, an expansion device forexpanding the heated, liquid agent, wherein an expanded, partiallyvaporized agent with a liquid phase and a vapor phase can be produced bypartial vaporization of the agent in the expansion device, and heatenergy in the agent can be converted to mechanical energy, a condenserfor condensation of the vapor phase of the partially vaporized agent inorder to produce the liquid agent, a separator for separation of theliquid phase from the vapor phase of the expanded, partially vaporizedagent, wherein the separator is arranged immediately before thecondenser in the flow direction of the agent, and is connected to thecondenser in order to supply the vapor phase to the condenser, and acombination means for combining the liquid phase and the condensed vaporphase of the partially vaporized agent, wherein the combination means isarranged before the pump in the flow direction of the agent and isconnected to the separator in order to supply the liquid phase, and tothe condenser in order to supply the condensed vapor phase, to thecombination means.
 6. The apparatus according to claim 5, wherein thepressure of the agent in the condenser can be set to an optimum betweenthe droplets of the liquid phase in the vapor phase of the agent beingas small as possible and the mechanical energy produced being as greatas possible in the expansion device.
 7. The apparatus according to claim5, wherein the combination means is in the form of an agent reservoir.8. The apparatus according to claim 5, wherein a nozzle and a turbineare arranged successively in the flow direction of the agent in theexpansion device.
 9. The apparatus according to claim 5, wherein thenozzle and the turbine form a single physical unit.
 10. The apparatusaccording to claim 5, wherein the low-temperature source is at atemperature of less than 400° C.
 11. A system for conversion of heatenergy from a low-temperature heat source to mechanical energy in aclosed circuit, comprising means for increasing the pressure of a liquidagent, a low-temperature heat source from which heat is transferred tothe increased-pressure, liquid agent without vaporizing the agent, meansfor expanding the heated, liquid agent, wherein an expanded, partiallyvaporized agent with a vapor phase and a liquid phase is produced bypartial vaporization of the agent, and heat energy in the agent isconverted to mechanical energy, and a condenser to condense the vaporphase in order to produce the liquid agent, wherein in the case of theexpanded, partially vaporized agent, the liquid phase is separated fromthe vapor phase immediately before the condenser and only the vaporphase is supplied to the condenser, the condensed vapor phase and theliquid phase are combined after the condenser in order to produce theliquid agent.
 12. The system according to claim 11, wherein the pressureof the agent in the condenser is set to an optimum between the dropletsof the liquid phase in the vapor phase of the agent being as small aspossible and the mechanical energy produced being as great as possible.13. The system according to claim 11, wherein the condensed vapor phaseand the liquid phase are combined in an agent reservoir.
 14. The systemaccording to claim 11, wherein the low-temperature source is at atemperature of less than 400° C.